Pipette graduated, types

The glassware used in titration is essentially the graduated type, that is, graduated flasks, pipettes, and burettes. The glassware is standardized and normalized. It is purchased bearing appropriate certificates. 

Pipettes and Petri Dishes. It is preferable to use presterilized disposable pipettes and petri dishes. They are not only more economical to use, but the method of sterilizing and sealing is generally better than that accomplished in most laboratories. Pipettes should be 1 ml serological type graduated in 0.1 ml. If concentrates are to be sampled, the large bore pipette for viscous liquids should be specified. Petri dishes should be standard 100 x 15 mm. 

Do not let the designation Class A mean more than it was meant to. Class A can only mean that it is the best tolerance readily available for that specific type of volumetric ware. Class A volumetric ware is not consistent across volumetric ware type. For example, a Class A volumetric pipette does not have the same degree of tolerance as a Class A measuring pipette. Equally, a Class A graduated cylinder does not have the same degree of tolerance as a Class A volumetric flask. See Table 2.6 for a representative cross comparison of Class A tolerances. 

Good Accuracy. Graduated pipettes and burettes should deliver within 1% of the nominal marked volume. There are several different types of calibration of graduated pipettes ... 

Pipettes of this type possess only one graduation mark and are used for delivery of that single volume of solution. Common sizes are 10, 20 and 50 mL. These pipettes are filled to a little above the mark by use of a pipette pump or a bulb. The pump is removed and the solution is allowed to run out until the mark is reached, the flow of solution being controlled all the way by use of the index finger over the end of the pipette. Most transfer pipettes are calibrated to allow a small volume of solution to remain in the tip of the pipette once it has been drained and no attempt should be made to blow this drop out of the bottom of the pipette. 

This type of transfer is slightly more complicated than the transfer of a colony from solid to liquid media hut is as important as these techniques are used to enumerate microorganisms. To perform these procedures, graduated pipettes are used to transfer specihc volumes to dilution blanks or to pour/spread plates (Section 14.3). Where reusable glass pipettes are used, these should be washed and placed into a metal canister for autoclaving and subsequent drying in an oven prior to use. Alternatively, sterilized disposable pipettes can be used for transfers (Section 18.7). 

Add 15.00 ml of sample to a 200-ml spoutless tail-form beaker by means of the automatic Knudsen-type pipette and add 15 ml of indicator-diluent solution. The titration is made from a Knudsen-type automatic bulb burette, the graduations of which generally commence at about 16.7. (This burette delivers twice the volume indicated by the graduations, to obtain an increased sensitivity.) The contents of the beaker must be vigorously stirred before and at the endpoint. It is best to commence stirring after the bulb has about two thirds emptied. About 1 ml before it is judged that the end point is reached rinse the sides of the beaker with a little distilled water from a wash bottle. 

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